The first thing to note when cleaning is that dust is one of the biggest enemies of the motherboard. It is best to pay attention to dust. You can use a brush to gently brush away the dust on the motherboard. In addition, some cards and chips on the motherboard are in the form of pins, which often lead to poor contact due to oxidation of the pins. You can use an eraser to remove the surface oxide layer and reconnect it. Of course we can use trichloroethane-it has good volatility and is one of the liquids for cleaning the motherboard.

Summary of methods for judging good or bad IC chips of PCB circuit boards!

1. Check board method:

1. Observation method: whether there is burnt, burnt, blistering, broken board surface, and corrosion of the socket.

2. Meter test method: Is the +5V and GND resistance too small (below 50 ohms).

3. Power-on inspection: For the board that is clearly broken, you can slightly increase the voltage 0.5-1V, and rub the IC on the board after power-on to let the chip in question heat up, so as to perceive it.

4. Logic pen check: check the signal for the input, output, and control poles of the suspected IC for the presence and strength of the signal.

5. Identify the major working areas: most boards have a clear division of labor on the area, such as: control area (CPU), clock area (crystal oscillator) (frequency division), background image area, action area (character, airplane), sound generation and synthesis District etc. This is very important for the in-depth maintenance of the computer board.

2. Troubleshooting method:

1. According to the instructions of the manual, first check whether there is a signal (wave type) at the input and output of the suspected chip. The possibility is extremely large, no control signal, trace to its previous pole until the damaged IC is found.

2. Don’t remove it from the pole for the time being found, you can choose the same model. Or put the IC with the same program content on it, turn it on and observe whether it is getting better, to confirm whether the IC is damaged.

3. Use wire cutting and jumper method to find the short-circuit wire: find that some signal wire and ground wire, +5V or other pins that should not be connected to the IC are short-circuited, you can cut off the wire and measure again to determine whether it is an IC problem or a board wiring problem , Or borrow signals from other ICs and solder them to the IC with the wrong wave pattern to see if the phenomenon picture becomes better, and judge the quality of the IC.

4. Comparison method: Find a good computer board with the same content to compare and measure the pin waveform and number of the corresponding IC to confirm whether the IC is damaged.

5. Use the ICTEST software in the microcomputer universal programmer (ALL-03/07) (EXPRO-80/100, etc.) to test the IC.

3. Chip disassembly method:

1. Foot cutting method: does not damage the board and cannot be recycled.

2. Drag tin method: Solder full tin on both sides of the IC pin, drag it back and forth with a high temperature soldering iron, and pick up the IC at the same time (the board is easy to damage, but the IC can be tested safely).

3. Barbecue method: Barbecue on an alcohol lamp, gas stove, electric stove, and IC will be released after the tin on the board has melted (difficult to master).

4. Tin pot method: Make a special tin pot on the electric stove. After the tin is melted, immerse the IC to be unloaded into the tin pot to pick up the IC without damaging the board, but the equipment is not easy to make.

5. Electric heating air gun: Use a special electric heating air gun to unload the film, blow the IC pin part to be unloaded, and then the IC after tinning can be lifted out.

As a professional hardware repair, board repair is one of the most important items. Bringing a faulty motherboard, how to determine which component is faulty?

The main causes of motherboard failure are:

1. Man-made faults: I/O cards are plugged in and removed under power, and improper force is used when installing boards and plugs, causing damage to interfaces, chips, etc.

2. Poor environment: Static electricity often causes chips on the motherboard (especially CMOS chips) to be broken down. In addition, when the motherboard encounters a power supply damage or an instantaneous spike generated by the grid voltage, it will often damage the chip near the power supply plug of the system board. If the motherboard is covered with dust, it will also cause signal short circuit and so on.

3. Device quality problems: damage caused by poor quality of chips and other devices.

The first thing to note when cleaning is that dust is one of the biggest enemies of the motherboard. It is best to pay attention to dust. You can use a brush to gently brush away the dust on the motherboard. In addition, some cards and chips on the motherboard are in the form of pins, which often lead to poor contact due to oxidation of the pins. You can use an eraser to remove the surface oxide layer and reconnect it. Of course we can use trichloroethane-it has good volatility and is one of the liquids for cleaning the motherboard.

Also, when the power is suddenly turned off, the computer should be shut down immediately to prevent the motherboard and power supply from being burned out by a sudden call. Process. BIOS Due to improper BIOS settings, if overclocking… you can jumper to clear the place, and reset it. If the BIOS is damaged, such as virus intrusion…, you can rewrite the BIOS. Because the BIOS cannot be measured by the instrument, it exists in the form of software. In order to eliminate all possible causes of the motherboard problems, it is best to flash the motherboard BIOS.

There are many reasons for the failure of the plug-in exchange host system. For example, the failure of the motherboard itself or the failure of various plug-in cards on the I/O bus can cause the system to operate abnormally. Using the plug-in repair method is a simple and convenient way to determine the fault in the motherboard or I/O device. The method is to shut down and pull out the plug-in boards one by one. Each time a board is pulled out, turn on the machine to observe the operating status of the machine. Once a certain board is pulled out, the main board is operating normally, then the cause of the failure is the plug-in board failure or the corresponding I/O bus slot And the load circuit is faulty.

If the system does not start normally after pulling out all the plug-in boards, the fault is likely to be on the motherboard. The exchange method is essentially to exchange the same type of plug-in boards, plug-in boards with the same bus mode and the same function, or chips of the same model with each other, and determine the fault location based on the change of the failure phenomenon. This method is mostly used in easy-to-plug maintenance environments, such as memory self-check errors, the same memory chip or memory bar can be exchanged to determine the cause of the failure. Take a look at a faulty motherboard and scan it with your eyes to see if there is no trace of burnout, whether the appearance is damaged, whether the plugs and sockets are skewed, whether the resistor and capacitor pins are touching, whether the surface is burnt, and the chip Whether the surface is cracked, and whether the copper foil on the main board is broken. Also check whether any foreign objects have fallen between the components of the motherboard. When you have questions, you can use a universal meter to measure it. Touch the surface of some chips. If it is abnormally hot, try another chip.

(1). If the connection is broken, we can use a knife to scrape the paint off the broken wire, apply wax to the exposed wire, and then use the needle to trace the wax away, and then drop the silver nitrate solution on it. Then use the universal meter to confirm whether the breakpoint is connected. Just connect the breakpoints one by one. Pay attention to the connections one by one, don’t be impatient, as the distance between the traces in some places on the motherboard is very small, and it will be short-circuited if it is not done.

(2). If it is an electrolytic capacitor, you can find a matching one to replace it. The universal meter and oscilloscope tools are used to Display the universal meter and wave device to measure the power supply of each component of the main board. One is to check whether the motherboard is supplying power to this part, and the other is to check whether the power supply voltage is normal. Resistance and voltage measurement: Power failures include +12V, +5V and +3.3V power supply and Power Good signal failures on the motherboard; bus failures include bus failures and bus control rights; component failures include resistors, capacitors, integrated circuit chips, and Failure of other components. To prevent accidents, you should measure the resistance between the power supply +5V and ground (GND) on the main board before powering up. The simplest method is to measure the resistance between the power pin of the chip and the ground. When the power plug is not inserted, the resistance should generally be 300Ω, and the minimum should not be less than 100Ω. Measure the reverse resistance value again, there is a slight difference, but the difference should not be too large. If the forward and reverse resistance values ​​are small or close to conduction, it means that a short circuit has occurred, and the cause of the short should be checked.

There are several reasons for this kind of phenomenon:

(1) There is a breakdown chip on the system board. Generally speaking, such faults are difficult to eliminate. For example, if the +5V of TTL chips (LS series) are connected together, the solder on the +5V pin can be sucked and suspended, and measured one by one to find out the faulty film. If the secant method is adopted, it will inevitably affect the life of the motherboard.

(2) There are damaged resistors and capacitors on the board.

(3) There are conductive debris on the board.

After removing the short-circuit fault, plug in all the I/O cards, and measure whether +5V, +12V and ground are short-circuited. Especially whether +12V collides with surrounding signals. When you have a good motherboard of the same model on hand, you can also use the method of measuring the resistance value to measure the suspicious points on the board. Through comparison, you can quickly find the chip failure. When the above steps are not effective, you can plug in the power supply and power on the measurement. Generally measure the +5V and +12V of the power supply. When it is found that a certain voltage value deviates too far from the standard, the voltage can be measured again by the separation method or cutting some leads or unplugging some chips. When a certain lead is cut or a certain chip is unplugged, if the voltage becomes normal, the component or the unplugged chip drawn by this lead is the fault.

Program and diagnostic card diagnosis Through random diagnostic program, special maintenance diagnostic card and according to various technical parameters (such as interface address), self-editing special diagnostic program to assist hardware maintenance can achieve a multiplier effect with half the effort. The principle of the program testing method is to use software to send data and commands, and to identify the fault location by reading the line status and the status of a certain chip (such as a register).

This method is often used to check various interface circuit failures and various circuits with address parameters. However, the prerequisite for the application of this method is the normal operation of the CPU and the base bus, the ability to run related diagnostic software, and the ability to run the diagnostic card installed on the I/O bus slot. The prepared diagnostic program should be strict, comprehensive and targeted*, can make certain key parts appear regular signals, can perform repeated tests on occasional failures, and can Display and record error conditions.

Fourth, the method of judging whether the IC integrated circuit is good or bad

1.Off-road detection

This method is carried out when the IC is not soldered into the circuit. Under normal circumstances, a multimeter can be used to measure the forward and reverse resistance values ​​of each pin corresponding to the grounding pin, and compare it with the intact IC. In-circuit detection This is a detection method that uses a multimeter to detect the DC resistance, AC and DC voltage to ground, and the total operating current of each IC pin in the circuit (the IC is in the circuit). This method overcomes the limitations of the substitution test method that requires a replaceable ic and the trouble of disassembling the ic. It is the most commonly used and practical method for detecting ic.

2.DC working voltage measurement

This is a way to measure the DC supply voltage and the working voltage of peripheral components with the DC voltage block of a multimeter when the power is on; detect the DC voltage value of each pin of the IC and compare it with the normal value to compress the fault range. Out of damaged components.

Pay attention to the following eight methods when measuring:

(1) The internal resistance of the multimeter should be large enough, at least more than 10 times the resistance of the circuit under test, so as to avoid large measurement errors.

(2) Usually turn the potentiometers to the middle position. If it is a TV, the signal source should be a standard color bar signal generator.

(3) Anti-slip measures should be taken for the test leads or probes. The ic is easily damaged due to any momentary short circuit. The following methods can be adopted to prevent the test pen from sliding: take a section of the bicycle valve core and put it on the tip of the test pen, and grow the tip of the test pen about 0.5mm, which can make the tip of the test pen well contact with the test point and effectively prevent slippage. , It will not short circuit even if it hits adjacent points.

(4) When the measured voltage of a certain pin does not match the normal value, it should be analyzed according to whether the pin voltage has an important influence on the normal operation of the ic and the corresponding changes in the voltage of other pins, and the quality of the ic can be judged.

(5) The voltage of the ic pin will be affected by peripheral components. When the peripheral components have leakage, short circuit, open circuit or variable value, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the sliding arm of the potentiometer will change the pin voltage.

(6) If the voltage of each ic pin is normal, it is generally considered that the ic is normal; if the voltage of the ic part of the pin is abnormal, start with the largest deviation from the normal value and check whether the peripheral components are faulty. If there is no fault, the ic is very likely damage.

(7) For dynamic receiving devices, such as TV sets, the voltage of each pin of ic is different when there is a signal. If it is found that the pin voltage should not change but the change is large, and the change with the signal size and the position of the adjustable component should not change, then the ic can be determined to be damaged.

(8) For devices with multiple working modes, such as video recorders, the voltage of each pin of ic is also different in different working modes.

3.AC working voltage measurement method

In order to grasp the change of the ic AC signal, a multimeter with a db jack can be used to approximate the ic’s AC working voltage. When testing, the multimeter is placed in the AC voltage block, and the positive meter pen is inserted into the db jack; for the multimeter without the db jack, a 0.1-0.5 μf DC blocking capacitor must be connected in series to the positive meter pen. This method is applicable to ICs with lower operating frequencies, such as the video amplifier stage of TV sets, field scanning circuits, etc. Because these circuits have different natural frequencies and different waveforms, the measured data is approximate and can only be used for reference.

4.Total current measurement method

This method is a method to judge whether the ic is good or bad by detecting the total current of the ic power supply line. Since most of the ICs are directly coupled, when the IC is damaged (such as a pn junction breakdown or open circuit), the subsequent stage will be saturated and cut off, and the total current will change. Therefore, the quality of ic can be judged by measuring the total current. It is also possible to measure the voltage drop of the resistance in the power path and use Ohm’s law to calculate the total current value.

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